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Magnetoreception

A bird’s magnetic compass has been demonstrated for fifty years, in laboratories all over the world. Nobody has found the receptor.

Animals steer by the Earth’s magnetic field — that much is about as well established as anything in behavioural ecology. What physically detects it has been argued over for fifty years and is still unresolved, which is an unusual state for a sense to be in: the behaviour is settled and the receptor is missing.

The Earth is a weak magnet. Its field is about a thousandth the strength of a fridge magnet at the surface, it passes through everything including bodies, and its direction and angle vary smoothly over the planet — which makes it, in principle, both a compass and a very coarse map. Animals use it. A caged migratory robin hops towards the heading it would fly; turn the field with coils and the hopping turns with it. That result is from 1972, it has been reproduced in laboratories across the world, and it has been extended to turtles, salmon, insects and others. The second finding from that first experiment is the one that makes it convincing: the robin’s compass reads the *angle* the field lines make with the ground rather than which end is north, which is why a bird carried across the magnetic equator behaves as though its compass has inverted. No other cue behaves like that. Then the problem. Fifty years on, nobody has identified the receptor. Two proposals compete — a light-dependent chemical reaction in the eye, and particles of magnetite acting as tiny compass needles — and each explains part of the evidence. The chemical account predicts that the compass should need light and run through the eye, and both hold. It also predicts that no cell has been found doing it, which is where matters stand. This page separates what is known about the behaviour from what is proposed about the mechanism, because the two are routinely reported at the same confidence and are not remotely at the same confidence.

Developed coverage · 88% complete · reviewed 2026-09-02

What this page covers

Magnetic orientation is demonstrated in migratory birds, sea turtles, salmonids, some insects, and — less securely — in bats, rodents and elasmobranchs. The mechanistic work is almost entirely on a handful of laboratory bird species.

Often confused with: Navigation in general, which uses the sun, stars, smell, landmarks and the magnetic field together

Quick facts

The behaviour
Established across birds, turtles, salmon and insects
The mechanism
Unresolved after fifty years; two proposals, no identified receptor
What a bird’s compass reads
The angle of the field lines, not which end is north
Compass or map?
Compass widely; map convincingly in one animal, on one route

The part that is not in doubt

A caged bird, a set of coils, and a result that has held up since 1972.

Diagram

How you ask a caged bird which way it wants to go

Schematic. Hop distributions are drawn, not measured.

How you ask a caged bird which way it wants to goEach mark is a hop.They cluster on a heading.Coils aroundthe cage set thefield to anythingyou like.Turn the field, andthe hops turn with it.A caged migrant hops towards where it would fly. That is the readout, and it isa proxy for a heading rather than a measurement of one.Schematic. Hop distributions are drawn, not measured.
The same explanation in words

Two circular orientation cages side by side. In the left cage, short marks radiate from the centre in a cluster pointing towards one heading — each mark is a hop, and a caged migratory bird hops repeatedly towards the direction it would fly if it could. Between the cages, a box explains that coils around the cage set the surrounding magnetic field to anything the experimenter chooses. In the right cage, the same cluster of hop marks now points in a different direction: turn the field, and the hops turn with it. A note beneath says that this hopping is a proxy for a migratory heading rather than a measurement of one.

That animals steer by the Earth’s magnetic field is not in doubt. What physically detects it has been argued for fifty years and is still unresolved.

Established

Specialists would state this without hedging. Multiple independent lines of evidence agree.

Magnetic compass orientation is demonstrated behaviourally across birds, sea turtles, salmonids, insects and other groups. No transduction mechanism is established: the light-dependent radical-pair proposal and magnetite-based proposals each account for parts of the evidence, and no receptor cell has been unambiguously identified in any vertebrate.

Who this applies to
The behavioural claim spans several groups; the mechanistic uncertainty applies to all of them.
Studied in
Aves, Reptilia, Actinopterygii, Insecta
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

Confidence is high in the split rather than in either half. The behaviour has been reproduced for five decades across laboratories and taxa; the mechanism has not been settled in the same period, and stating both plainly is the accurate position.

How far it can be extended

Independent orientation-cage and field manipulations in unrelated lineages produce compass responses to controlled field changes.

Caveats

  • Magnetite-based and radical-pair proposals are not mutually exclusive; an animal could have both, doing different jobs.
  • Orientation-cage behaviour is a proxy for migratory heading rather than a measurement of it.
  • Some magnetic-effect results have proved difficult to replicate between laboratories, which is itself informative about effect sizes.

Still unanswered

  • Which molecule or structure actually transduces the field in any vertebrate — the central unsolved problem of the field.

Last reviewed 2026-09-02

The evidence (3 studies)

The detail that turns this from suggestive into decisive is what the compass reads. Reverse the horizontal component of the field and the bird reorients, as you would expect. Reverse the *vertical* component — which flips the angle the field lines make with the ground without swapping magnetic north and south — and it also reorients. Reverse polarity alone and nothing happens. A bird’s compass therefore does not distinguish north from south at all; it distinguishes poleward from equatorward by the steepness of the field lines. Nothing else in a laboratory behaves that way, which is why the result has survived every attempt to explain it as a response to something correlated with direction.

A bird’s magnetic compass tells poleward from equatorward, not north from south

Established

Specialists would state this without hedging. Multiple independent lines of evidence agree.

European robins tested in coil systems reoriented when the horizontal component of the magnetic field was turned, but reversing the horizontal component alone did not reverse their heading; reversing the vertical component did. The compass therefore responds to the inclination of the field lines relative to gravity rather than to magnetic polarity, distinguishing poleward from equatorward directions.

Who this applies to
night-migrating songbirds, established in the European robin
Studied in
Erithacus rubecula, Aves

You may have heard

“Birds follow the Earth’s magnetic field like a GPS”

Two errors in one sentence. A compass gives direction and GPS gives position — knowing which way is poleward tells a bird nothing about where it is. And the bird’s compass is not the kind in a hiker’s pocket: it reads the angle the field makes with the ground rather than which end points north.

Why we rate it this way, and what the caveats are
EstablishedHigh confidence

A direct manipulation with the diagnostic control: separating the horizontal and vertical components tells the two candidate mechanisms apart, and the result is unambiguous.

How far it can be extended

The inclination compass has since been demonstrated in many migratory bird species; it is not universal among animals with magnetoreception.

Caveats

  • Cage orientation measures intention rather than a flight path.
  • The receptor mechanism remains contested, with cryptochrome and magnetite hypotheses both unresolved.
  • An inclination compass gives no useful signal at the magnetic equator, where the field lines are horizontal.

Still unanswered

  • Which receptor actually detects the field, and where is it?

Last reviewed 2026-08-10

The evidence (2 studies)
Words used here
Inclination
The angle the magnetic field lines make with the ground. Steep near the poles, flat at the magnetic equator — which is what an inclination compass measures.
Orientation cage
A circular cage that records which way a caged migratory bird hops. During migration season the hopping is directional, which makes it a usable readout of intended heading.

The part nobody has solved

Two proposals, fifty years, and no receptor cell identified in any vertebrate.

Diagram

The leading idea, and it is an idea rather than a finding

The leading idea, and it is an idea rather than a finding1 · a photon2 · two molecules3 · the field, at an angle4 · a signal thatvaries with headingWhat it predicts, and what has been checkedNeeds light — checked, and holds. Runs through the eye — checked, holds.Reads the angle, not north from south — checked, holds.No molecule in any living bird has been shown to do this.Schematic. Nothing here is drawn to scale and no receptor has been identified.
The same explanation in words

A four-step chain. First, a photon of light is absorbed. Second, that absorption creates two molecules each carrying an unpaired electron — a radical pair. Third, the surrounding magnetic field, drawn as a set of parallel lines at an angle, influences how those two molecules behave, and the influence depends on the angle between them. Fourth, the result is a signal that varies with the direction the animal is facing. Beneath, the predictions are listed against what has been checked: the compass should need light, which holds; it should run through the eye, which holds; and it should read the angle of the field rather than distinguishing north from south, which holds. A final line in a warning colour states that no molecule in any living bird has been shown to do this.

The leading idea is that a bird’s compass is chemical and needs light: absorbing a photon creates a pair of molecules whose behaviour depends on the angle of the surrounding magnetic field.

Emerging evidence

Real findings exist, but too few or too recent to be settled.

The radical-pair hypothesis proposes that photon absorption generates a spin-correlated radical pair, most likely in a retinal cryptochrome, whose singlet–triplet interconversion rate varies with the orientation of the external field. It predicts light dependence, axial rather than polar sensitivity, and disruption by weak radio-frequency fields at specific resonances.

Who this applies to
Proposed principally for birds; extensions to other groups are speculative.Do not extend this beyond the taxa listed — the popular version over-reaches.
Studied in
Aves
Why we rate it this way, and what the caveats are
Emerging evidenceModerate confidence

The model has an unusually good record at surviving its own predictions — light dependence and eye dependence both hold. It remains a model: no cryptochrome has been shown to perform this function in a living bird, and the radio-frequency results have replication difficulties.

How far it can be extended

The supporting behavioural evidence is almost entirely from a small number of migratory passerines tested in orientation cages.

Caveats

  • Cryptochrome is a candidate host molecule; no specific cryptochrome has been demonstrated to transduce the field in a bird.
  • The eye dependence and lateralisation results are less consistent across laboratories than early reports suggested.
  • A model that survives its predictions is a good model, not a finding.

Still unanswered

  • Whether any cryptochrome in the avian retina has the lifetime and geometry the model requires.

Last reviewed 2026-09-02

The evidence (3 studies)

How we know

Covering one eye of a bird trying to migrate

If a bird’s magnetic compass runs through its eyes, does it matter which eye?

Migratory robins were placed in circular orientation cages during the season when a caged migrant hops persistently towards the direction it would fly if it could. The direction of that hopping was recorded three times for each bird: with a small opaque cap over the left eye, with the cap over the right eye, and with both eyes uncovered. Nothing about the magnetic field was altered — the only thing that changed was which eye could see.

What happened

Birds using only the right eye oriented as accurately as birds using both. Birds restricted to the left eye scattered — no consistent direction at all.

What it shows

The compass depends on the eyes, which is a far stronger statement than it sounds. A magnetic sense could in principle live anywhere in the body; a magnetic sense that stops working when you cover one eye is being read somewhere in the visual system, which is the central prediction of the light-dependent account.

What it does not show

It does not show what the receptor is, and it does not show that the bird sees anything. A dependence on the eye locates the pathway, not the molecule. The lateralisation itself has also proved fragile: later studies have found it in some age groups and not others, and some laboratories have not found it at all, so this is a result to cite for the eye dependence rather than for the asymmetry.

The controls — what makes this evidence rather than a story
  • Both-eyes-open trials for every bird, so each animal is its own baseline.
  • The cap on each side in turn, so an effect cannot be a response to wearing a cap.
  • The magnetic field left untouched throughout, isolating the eye rather than the stimulus.

From Lateralization of magnetic compass orientation in a migratory bird

The other proposal is mechanical rather than chemical: crystals of magnetite, which is naturally magnetic, embedded in tissue and pulled on by the field like a compass needle pulling on its bearing. Magnetite is genuinely present in some animals, and there is a plausible route for the signal in the trigeminal nerve of birds. What has not been produced is an identified receptor cell — and one much-cited candidate turned out, on close examination, to be iron-rich immune cells rather than sensory ones. The two proposals are also not rivals for one job: a chemical compass in the eye and a magnetite intensity detector elsewhere could coexist in the same bird doing different things, and several researchers think they do.

Knowing which way is north is not knowing where you are

Two different problems, with very different amounts of evidence behind them.

Diagram

Two different problems, and two very different amounts of evidence

Schematic. The grid is not a real field map.

Two different problems, and two very different amounts of evidenceA compassWhich way am I pointing?Established across birds, turtles, fish, insects.A mapWhere am I?Best evidence: naive turtles, one route.The field gives direction almost everywhere. Getting position out of it needsgradients read finely enough to tell one place from another — much harder.Schematic. The grid is not a real field map.
The same explanation in words

Two panels. The left, outlined in green and headed "A compass", asks "Which way am I pointing?" and shows a needle in a circle. Beneath it: established across birds, turtles, fish and insects. The right, outlined in a warning colour and headed "A map", asks "Where am I?" and shows a grid of points with one highlighted. Beneath it: the best evidence is naive turtles, on one route. A closing note says the field gives direction almost everywhere, while getting position out of it requires reading gradients finely enough to tell one place from another, which is much harder.

Knowing which way is north is a different problem from knowing where you are, and the evidence for the two is very different in strength.

Well supported

Good evidence backs this, though some details remain open.

Compass mechanisms provide directional information from field inclination or polarity. Map mechanisms would require position to be inferred from spatial gradients in field intensity and inclination. Compass orientation is demonstrated widely; positional use of the field is demonstrated in far fewer systems, most convincingly in naive sea turtles responding to regional field signatures.

Who this applies to
The compass claim is broad; the map claim rests principally on hatchling sea turtles and on a smaller body of work in birds and fish.Do not extend this beyond the taxa listed — the popular version over-reaches.
Studied in
Caretta caretta, Aves
Why we rate it this way, and what the caveats are
Well supportedHigh confidence

The distinction itself is not disputed and is the organising idea of the navigation literature. The confidence attaches to the separation, not to any particular animal having a map.

How far it can be extended

Extending the turtle map result to animals generally would credit them with positional information that has not been demonstrated.

Caveats

  • The turtle result is a set of inherited responses at a handful of positions, which is closer to signposts than to coordinates.
  • Field intensity and inclination change over decades, so any inherited map would need updating or recalibrating.
  • Displacement experiments in birds are hard to interpret because so many other cues are available.

Still unanswered

  • Whether any animal derives true position from the magnetic field rather than recognising a small number of learned or inherited signatures.

Last reviewed 2026-09-02

The evidence (3 studies)

How we know

Putting a newly hatched turtle in a magnetic field from a thousand miles away

Does a turtle that has never left the beach already know what to do at a place it has never been?

Loggerhead hatchlings, straight from the nest and with no experience of the open sea, were tethered to a swivel arm in a circular tank so that their swimming direction could be recorded without letting them go anywhere. The tank sat inside a coil system that could reproduce the exact strength and inclination of the Earth’s field as it is at specific points around the North Atlantic gyre — the current the animals spend their first years riding. Each turtle was tested in several of these simulated locations.

What happened

The turtles swam in different directions in different simulated fields, and in each case the heading was one that would have kept a real turtle inside the warm gyre rather than carrying it out into cold water.

What it shows

Positional information, not just directional. A compass tells an animal which way is north; this tells it something about where it is, because the response depends on which field it is sitting in. And because these animals had never been anywhere, the responses cannot have been learned — a turtle hatches already carrying a small set of instructions keyed to magnetic coordinates.

What it does not show

It is not a map in the sense of a turtle knowing its position. What was demonstrated is a set of inherited reflexes at a handful of points along one route in one species — closer to a few signposts than to a coordinate system. The animals were also tethered in a tank with every other cue removed, and a real hatchling uses light and wave direction first.

The controls — what makes this evidence rather than a story
  • Hatchlings with no migratory experience, so any location-specific response has to be inherited rather than learned.
  • Several simulated locations per animal, so a fixed preference would show up as the same heading everywhere.
  • The tank in darkness with the coils the only directional cue available.

From Regional magnetic fields as navigational markers for sea turtles

Hatchling turtles that have never migrated still swim the right way for where they are

Well supported

Good evidence backs this, though some details remain open.

Loggerhead hatchlings (Caretta caretta) exposed to magnetic fields replicating specific points on their migratory route oriented in directions appropriate to those locations, without prior experience of them.

Who this applies to
loggerhead sea turtle hatchlings, tethered in a laboratory coil system
Studied in
Caretta caretta
Why we rate it this way, and what the caveats are
Well supportedHigh confidence

The animals were naive, which removes learning as an explanation, and orientation tracked the simulated location rather than any fixed direction.

Caveats

  • Tethered hatchlings in a tank; the available behaviour is constrained.
  • Shows position-appropriate orientation, not that the animal knows where it is.
  • Field manipulations are coarser than the gradients a turtle meets in a short journey.

Still unanswered

  • Is an inherited response set a map, or a very good lookup table?

Last reviewed 2026-08-09

The evidence (2 studies)

Do birds see magnetic field lines?

The most repeated sentence about this sense, and the least supported.

The short answer

Can birds see the Earth’s magnetic field?

Their compass appears to run through the eyes and to need light, which is a real and surprising finding. That it produces something a bird sees is not a finding — it is an illustration of a model, and no experiment has addressed it.

The imagery comes from the mechanism rather than from any observation. If the compass works by a light-triggered chemical reaction spread across the retina, then its output would vary across the visual field as the bird turns its head, and artists have drawn that as a shimmer or a set of glowing lines superimposed on the world. The drawing is a way of picturing what the model implies about a signal. It is not a claim anybody has tested, and it could not be: nothing in an orientation cage distinguishes a bird that perceives a magnetic pattern from a bird that is steered by one without noticing. This is not an argument that birds experience nothing. It is that the question has not been touched by the evidence usually offered for it, and the honest answer is that we do not know.

Check it for yourself

A bird’s compass appears to run through its eyes and to need light. That is not the same as the bird seeing the magnetic field, and nothing has shown that it does.

Not enough evidence

Nobody has done the work needed to answer this properly.

Evidence for light dependence and ocular involvement in avian magnetoreception constrains the transduction pathway. It does not establish a visual percept: no experiment distinguishes a modulation of visual processing that the animal experiences from one it does not.

Who this applies to
Birds, where the light-dependent evidence is concentrated.Do not extend this beyond the taxa listed — the popular version over-reaches.
Studied in
Aves
Why we rate it this way, and what the caveats are
Not enough evidenceHigh confidence

High confidence that the claim is unsupported rather than that it is false. Subjective experience is not accessible to any of these designs, and the popular version asserts something no experiment has addressed.

How far it can be extended

The ocular evidence is specific to birds; extending a visual percept to turtles, fish or insects has no support at all.

Caveats

  • This is not an argument that birds experience nothing — it is that the question has not been addressed by any of the evidence usually cited for it.
  • Illustrations of birds seeing glowing field lines are visualisations of a model, not depictions of a finding.

Still unanswered

  • Whether any experimental approach could distinguish a magnetic signal the animal perceives from one that steers it without being perceived.

Last reviewed 2026-09-02

The evidence (3 studies)

Where else it turns up

  • Sea turtles: hatchlings respond to regional field signatures before they have been anywhere, which is the strongest map evidence in any animal.
  • Salmon: returning adults appear to use field signatures learned or inherited for their natal river, though the evidence is largely correlational across decades of catch data.
  • Insects: monarch butterflies use a light-dependent inclination compass alongside a time-compensated sun compass, and bees and ants show magnetic effects on orientation.
  • Bats and rodents: real evidence, smaller and thinner, and in bats apparently calibrated against the sunset sky rather than used alone.
  • Sharks and rays: often asserted on the basis of their electroreceptors, and the behavioural demonstrations are much weaker than the anatomy suggests they should be.

And the wider point: no migratory animal relies on the magnetic field alone.

Navigating animals use several overlapping compasses, not a single magic sense

Established

Specialists would state this without hedging. Multiple independent lines of evidence agree.

Migratory animals orient using redundant mechanisms — sun compasses with circadian time compensation, star patterns, polarised light, olfactory cues and magnetic field direction — with different species weighting them differently and switching when one is unavailable.

Who this applies to
the migratory and homing species in which orientation has been directly tested
Studied in
Danaus plexippus, Caretta caretta, Apis mellifera, Cataglyphis fortis
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

Compass orientation is among the best-replicated findings in animal behaviour, demonstrated independently in insects, birds and reptiles using manipulations that isolate each cue.

How far it can be extended

Assembled from primary work in each species. The claim is about the redundancy of mechanisms across lineages, built only from cases with their own evidence.

Caveats

  • Knowing which way is north is not knowing where you are — compass and map are different problems.
  • The mechanism of magnetic detection remains unresolved despite decades of work.
  • Evidence concentrates in a few well-studied species.

Still unanswered

  • How is the magnetic field actually detected?
  • Which animals, if any, have a true positional map?

Last reviewed 2026-08-09

The evidence (4 studies)

Where this connects

What is still unsettled

  • What molecule or structure actually transduces the field?

    Why it matters: It is the central unsolved problem of the field, and until it is answered every mechanistic statement about magnetoreception is a proposal.

    What would settle it: Identifying a cell whose response to a controlled field change can be recorded, and whose removal abolishes the behavioural compass.

  • Do animals derive true position from the field, or recognise a small number of signatures?

    Why it matters: It is the difference between a map and a set of signposts, and popular accounts describe the first while the evidence supports the second.

    What would settle it: Displacement experiments to positions with no plausible inherited or learned signature, which are much harder to design than they sound.

  • Why are magnetic-effect results so much harder to replicate than most behavioural findings?

    Why it matters: Several results in this field — the lateralisation, the radio-frequency disruption — have appeared in some laboratories and not others. That pattern usually means small effects, hidden variables, or both.

    What would settle it: Pre-registered multi-laboratory replication of the standard designs, which has been called for repeatedly and rarely done.

Claims about this, checked

Things people have heard, and what the evidence actually supports.

The research behind this page

12 studies, newest first. Each one has a page explaining what it found and what it could not show.

2018Nature

Long-distance navigation and magnetoreception in migratory animals

Navigation in migratory animals is multi-sensory and redundant: compasses of several kinds are well established, while the mechanism of the magnetic sense and the existence of a true positional map remain unresolved.

2015Current Biology

Eurasian Reed Warblers Compensate for Virtual Magnetic Displacement

Birds shifted their headings in the direction that would compensate for the displacement the simulated field implied, despite no other cue having changed.

2015Current Biology

Evidence for Geomagnetic Imprinting and Magnetic Navigation in the Natal Homing of Sea Turtles

Where magnetic signatures of adjacent coastal areas converged over time, nesting density increased; where they diverged, it decreased.

2013Oxford University Press

Sensory Ecology, Behaviour, and Evolution

Sensory systems are shaped by the physics of the environment and by the specific tasks an animal performs, and no sensory system is general-purpose: sensitivity in one dimension is routinely traded against resolution, speed or energy in another.

2010Trends in Neurosciences

Navigational mechanisms of migrating monarch butterflies

Monarchs use a time-compensated sun compass whose clock sits in the antennae rather than the brain; removing or painting the antennae disrupts orientation while leaving flight intact.

2006Science

The ant odometer: stepping on stilts and stumps

Ants on stilts overshot the nest; ants with shortened legs stopped short.

2005Proceedings of the National Academy of Sciences

Honey bees navigate according to a map-like spatial memory

Displaced bees often flew an initial vector, then changed course and headed directly to the hive or feeder, suggesting they could relate their position to remembered locations.

2004Nature

Geomagnetic map used in sea-turtle navigation

Turtles exposed to the field of a northern site swam south, and those exposed to a southern field swam north — in each case towards their actual home area.

2002Nature

Lateralization of magnetic compass orientation in a migratory bird

Birds using the right eye oriented normally.

2001Science

Regional magnetic fields as navigational markers for sea turtles

Turtles swam in directions appropriate to the location whose magnetic signature they were exposed to, orienting differently for fields matching different points on the route — without ever having been there.

2000Biophysical Journal

A model for photoreceptor-based magnetoreception in birds

The model predicts a magnetic effect that is light-dependent, direction-sensitive but not polarity-sensitive, disrupted by weak radio-frequency fields at specific resonances, and modulated across the visual field as the head turns.

1972Science

Magnetic compass of European robins

Birds reoriented when the horizontal component was turned, showing they use the magnetic field for direction.

This page is a stop on a longer route

A guided journey reads several subjects in a deliberate order, with an argument for why one follows another. You can join in the middle.

Where to go from here

Each of these follows from something on this page — a relationship in the evidence, a claim people ask about, or the next mechanism along.

How complete this page is, and what it is still missing

NatureHQ publishes its own gaps. This page is at 88% completeness against what we would call a finished subject, and was last reviewed on 2026-09-02. It carries 10 claims and answers 22 mapped search questions.

  • no research from the last few years is attached — check for newer work
  • The magnetite proposal is summarised in a paragraph and deserves the same treatment as the radical-pair account.
  • Salmon and elasmobranch magnetoreception are named rather than assessed.
  • Nothing here covers how magnetic information is combined with the sun and star compasses, which the navigation page treats and this one defers to.